Reactions of period 3 elements with chlorine

The period 3 elements, except noble gas argon, react with chlorine gas in exothermic reactions to form chlorides. The reactions are generally vigorous, occurring rapidly at room temperature.

Sodium - Reacts vigorously when heated and plunged into chlorine gas, forming white sodium chloride.

2Na_(s) + Cl_2(g) ➔ 2NaCl_(s)

Magnesium - Reacts vigorously with chlorine gas to produce magnesium chloride.

Mg_(s) + Cl_2(g) ➔ MgCl_2(s)

Aluminium - Reacts vigorously with excess chlorine gas, forming white aluminium chloride.

2Al_(s)_ + 3Cl_2(g)_ ➔ Al_2_Cl_6(s)_

Silicon - Reacts slowly with chlorine gas at room temperature to produce silicon(IV) chloride liquid.

Si_(s)_ + 2Cl_2(g)➔ SiCl_4(l)

Phosphorus - Reacts slowly when exposed to excess chlorine, forming liquid phosphorus pentachloride.

2P_(s)_ + 5Cl_2(g)_ ➔ 2PCl_5(l)_

Oxidation numbers in period 3 chlorides

The table below shows the common chloride compounds formed by period 3 elements, along with their oxidation numbers:

The oxidation number of the period 3 element in the chloride increases by one as you move across the period. This happens because:

• The period 3 elements fill their outer shell sequentially with electrons when bonding to chlorine.
• With each step across, one more valence electron is used in bonding to chloride. The period 3 elements exist in positive states in the chloride because chlorine has the highest electronegativity of any period 3 element.

Effect of water on period 3 chlorides

The table below summarises the bonding, structure and reactions with water for various period 3 chlorides.

The reactivity of the period 3 chlorides with water changes across the period due to differences in structure and bonding.

Sodium and magnesium chlorides

The ionic chlorides NaCl, MgCl₂ dissolve in water without reaction because they have giant ionic lattice structures. When dissolved in water, these lattices break apart into separate hydrated ions which are stabilised by surrounding polar water molecules.

The equations are:

• NaCl_(s) ➔ Na⁺_(aq) + Cl^-_(aq)
• MgCl_2(s) ➔ Mg²⁺_(aq) + 2Cl^-_(aq)

Aluminium chloride

The covalent metallic chloride Al₂Cl₆ reacts with water to form an acidic solution because it consists of simple covalent molecules with strong shared electron pairs holding the atoms together.

Al₂Cl₆ is a covalently-bonded molecule - it can be thought of as a dimer of AlCl₃. Each Al forms 4 bonds to Cl atoms - 3 single covalent bonds and 1 dative covalent bond.

On reaction with water, the AlCl₃ dimers break apart into Al³⁺ ions which strongly attract water molecules, causing them to lose H⁺ ions that give an acidic solution:

The equation is:

• [Al(H₂O)₆ ]³⁺_(aq) ➔ [Al(H₂O)₅OH]²⁺_(aq) + H⁺_(aq)

Silicon and phosphorus chlorides

The covalent non-metal chlorides SiCl₄ and PCl₅ rapidly hydrolyse releasing hydrogen chloride gas because they have simple covalent structures. When water is added, the polar water molecules break the polar covalent Si-Cl and P-Cl bonds, as new Si-O, P-O and H-Cl bonds start forming rapidly.

This releases HCl gas, shifting the equilibrium towards the products.

The equations are:

• SiCl_4(l) + 2H₂O_(l) ➔ SiO_2(s) + 4HCl_(g)
• PCl_5(s) + 4H₂O_(l) ➔ H₃PO_4(aq) + 5HCl_(g)